The stock market is running out of gas—but if the Federal Reserve indicates it will keep interest rates low, price-to-earnings ratios could head higher. Small stocks, however, look ready for a fall.

During a game of Jenga, there comes a moment when just one block supports the entire edifice and pulling it out causes the entire tower to tumble. Has the market reached that moment?

Stocks sure felt wobbly last week. The Standard & Poor's 500 dropped 2% to 1841, as tensions in Ukraine heated up and weak economic data out of China rekindled concerns of a hard landing in the world's second-largest economy. Yet for all the Sturm und Drang, the S&P 500's tumble looks modest compared with declines suffered by Germany's...